REFACTOR: Remove the rest of the `Discourse` constants in our application (PR #10480)

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Does this need to switch from const to let?

No it doesn’t. let vs const is not really a big deal and IMO is mostly a preference. I used to be bullish on it, making everything const, but eventually I came around when I realised that almost no bugs are prevented. It doesn’t ensure immutability of the object, only re-assignment. And JS doesn’t make any optimizations based on it currently.